This invention relates to the remote guidance of vehicles, and, more particularly, to a method for guiding a vehicle using two global positioning satellite system receivers to provide relative guidance and reduce positioning errors.
There are several basic techniques for guiding vehicles, typically flight vehicles, to their targets or destinations. In the most common, the vehicle itself has an onboard sensor that makes sensor contact with ("acquires") the target. A vehicle controller then steers the vehicle to the acquired target. This approach works well in many contexts, where the on-board sensor can actually make initial contact with the target and can provide sufficient information for guidance.
The on-board sensor approach becomes less satisfactory where attempts are made to avoid acquisition of the target, as by hiding it. In that case, more information may be needed than can be provided by the on-board sensor, leading to tile use in the guidance of information from other sources. The approach of relying on on-board sensors may also not work close to the ground when the sensor field is cluttered, or where tile data provided by the sensor is not sufficiently precise.
Technical attributes of the sensor must also be viewed in relation to its cost. In the case of precision guided munitions, guided by light, infrared, or radar sensors, the cost of the sensor and its electronics is a significant fraction of the cost of the vehicle. The more precise the sensor, the higher its cost.
With these technical considerations and the system costs in mind, techniques for guiding vehicles to their destinations or targets using information from remotely positioned controllers or sensors have been developed. In a civilian context, an all-weather aircraft landing system may use, in part, remotely generated navigational information to guide an aircraft to a safe landing even in a near total absence of visibility. In a military context, precision guided weapons can be guided to their targets by using a sensor on a targeting aircraft to locate a target, and providing the location of the target to a weapon launched by the aircraft. Increasingly sophisticated data links have made it possible to use a variety of remotely generated information in guiding precision munitions and missiles to their targets. These techniques reduce (or eliminate) the sensor costs of the weapons themselves, thereby significantly reducing the disposable cost of the weapon system.
One guidance approach that has been suggested for both civilian and military remotely guided vehicles utilizes the global positioning system (GPS). The GPS provides a number of satellites in orbit above the earth, each satellite emitting one or two navigational signals. The GPS satellites are arranged so that there will always be several satellites in the field of view of any pertinent place on the earth. The precise location of that point can be fixed by measuring the time required for the navigational signal of three, or preferably four, of the satellites to reach that point, in a variant of a triangulation approach. The GPS system is largely unaffected by weather, and, in the military context, is not affected by many camouflage techniques.
The GPS system is in operation, and low-precision GPS receivers are available for as little as about a thousand dollars for use by individuals. Higher precision GPS receivers are used in civilian and military applications. Depending upon the precision of the GPS receiver chosen, the GPS system allows the determination of absolute position to within a certainty of about 30 feet at most locations on the earth. This degree of certainty means that there is a specified high probability that the lndicated location is within 30 feet of the correct location, and is known as the circular error probability (CEP).
GPS-based guidance systems have been proposed for use in aircraft landing systems and guided munitions. Unfortunately, in both of these applications the lndicated 30 foot CEP is too great to be practical in most instances. A 30 foot error in the altitude of the runway in an aircraft landing system can lead to disaster. A miss of 30 feet by many precision guided munitions can result in failure of the mission to achieve its objectives.
This problem has been to some extent solved for landing systems and other civilian applications by analyzing the nature of the inherent GPS error. The greatest part of the error arises from bias-type, systematic errors. Examples of error sources are slight uncertainties in knowing the precise positioning of the satellites, slight errors in the satellite clock, and signal variations caused by atmospheric conditions. These errors identically affect all GPS receivers within an area. They can be accounted for by locating a fixed GPS receiver at a surveyed place whose true location is known precisely (e.g., the end of the runway), measuring the range of that fixed receiver to the satellites in view in GPS coordinates, and comparing the measured ranges with the true ranges determined from the known location to obtain correction values for each satellite. These correction values are broadcast to mobile GPS systems in the area, which then track the satellites that yield the best positional information. The ranges determined by the mobile systems in GPS coordinates are corrected by the correction values broadcast by the fixed receiver. With this "differential GPS" technique, the absolute position error using GPS can be reduced to less than 10 feet CEP.
The differential GPS approach would be operationally unsuited for many military targeting applications, many other military applications, and many civilian applications. In these cases, a GPS receiver cannot be placed at an accurately surveyed location whose true position is known, to provide a measurement of the bias-type error corrections.
There is therefore a need for an improved technique for providing remote navigational and guidance information for use in both civilian and military applications. The present invention fulfills this need, and further provides related advantages.